Methods and circuits employing threshold voltages for mask-alignment detection

ABSTRACT

Described are mask-alignment detection structures that measure both the direction and extent of misalignment between layers of an integrated circuit. Each structure includes one or more MOS transistors, each of which exhibits a threshold voltage that varies with misalignment in one dimension. The test structures are configured in mirrored pairs, so that misalignment in one direction oppositely affects the threshold voltages of the paired structures. The threshold voltages of the paired structures can therefore be compared to determine the extent and direction of misalignment. Measurements in accordance with the invention are relatively insensitive to process variations, and the structures using to take these measurements can be formed along with other features on an integrated circuit using standard processes. One embodiment of the invention may be used to measure misalignment between active implants and the windows in which active regions are formed. Other embodiments measure misalignment between threshold-voltage implants and the active regions.

FIELD OF THE INVENTION

The invention relates to semiconductor fabrication, and in particular tomask-alignment test structures for measuring the alignment ofsuperimposed elements formed on and within a semiconductor layer.

BACKGROUND

Most semiconductor devices are built up using a number of materiallayers. Each layer is patterned to add or remove selected portions toform circuit features that will eventually make up a complete integratedcircuit. The patterning process, known as photolithography, defines thedimensions of the circuit features.

The goal of the patterning process is to create circuit features in theexact dimensions required by the circuit design and to place them in theproper locations on the surface of a semiconductor wafer. Perfectalignment is an ideal that cannot be achieved in practice. Instead, thevarious layers of an integrated circuit are misaligned to some extent.Such misalignment is termed “mask misalignment” because misaligned maskimages are the source of the error. When circuits fail duringfabrication, it is desirable to determine whether the source of thefailure is incorrect mask alignment.

There are a number of conventional methods of detecting maskmisalignment. For example, U.S. Pat. No. 5,770,995 to Masayuki Kamiyadescribes a structure that identifies misalignment between a conductivelayer and a contact window layer. The disclosed structure indicates thedirection of mask misalignment but does not provide an accurate measureof the extent of misalignment. Each of U.S. Pat. No. 4,386,459 to DavidBoulin and U.S. Pat. No. 4,571,538 to Pei-Ming Chow describe structuresthat indicate both the direction and extent of mask misalignment. Theabove-mentioned U.S. Patents provide useful background information, andare therefore incorporated herein by reference.

FIG. 1A is a plan view of mask regions used to define a conventional MOStransistor 100. FIG. 1B depicts MOS transistor 100 in cross section,taken along line A-A′ of FIG. 1A. A window 105 in an insulating layer110 (FIG. 1B) serves as a mask to form the active regions of MOStransistor 100. Window 105 might be formed, for example, along withsimilar windows in a field-oxide layer used to define active regions ina standard CMOS process. The term “active region” refers here to thearea encompassing the source, drain, and channel regions of MOStransistor 100 in particular, and MOS transistors in general.

Modern semiconductor processes allow for precise adjustment oftransistor threshold voltages. Threshold-voltage adjustments are made byaltering the conductivity of transistor channel regions by implantingrelatively low concentrations of dopants. N-channel and P-channeltransistors require different dopant types and concentrations, so masksare used to expose the target areas and to shield other areas frominappropriate implants. These masks must therefore be properly aligned.FIG. 1A depicts the boundary 111 of an exemplary threshold-voltageimplant; FIG. 1B depicts a threshold-voltage implant 112.

After formation of threshold-voltage implant 112, a gate 115, and a gateinsulator 120 are formed over the region defined within window 105.Dopant atoms are then implanted in window 105 to create the source anddrain regions. FIG. 1A depicts the boundary of an exemplaryactive-region implant 121; the mask used to define boundary 121 must bealigned with window 105. Gate 115 —typically polysilicon—masks theunderlying substrate 117, thus defining a channel region 122 betweensource and drain regions 125 and 130. In modern CMOS processes, theactive-region implant largely defines the dopant level in gate 115, sothat the conductivity type of gate 115 matches that of source and drainregions 125 and 130 for both PMOS and NMOS transistors.

Misalignment of the threshold-voltage implant mask produces very littlechange in resistance, and is therefore difficult to measure usingalignment-measuring schemes that depend upon variations in resistance.Misalignment of the active-region implant is also difficult to measureusing resistive means because salicide formations in the active regionsgreatly reduce the sheet resistance of the active regions, and thereforeobscure resistance variations that result from misaligned active-implantmasks. There is therefore a need for a mask-alignment detectionstructure that accurately indicates the direction and extent ofmisalignment for circuit features that produce little resistivevariation when misaligned.

SUMMARY

The present invention satisfies the need for an accurate mask-alignmentdetection structure that measures both the direction and extent ofmisalignment between features of an integrated circuit. Measurementstaking using structures in accordance with the invention are relativelyinsensitive to process variations, and the test structures can be formedalong with other features on an integrated circuit using standardprocesses.

One embodiment of the invention measures the extent to whichactive-region implants are aligned with the areas on a semiconductorsubstrate in which the active regions are to be formed. One teststructure in accordance with that embodiment is an MOS transistor thatconventionally includes source and drain regions separated by a gate. Apair of active-region implants of a first conductivity type defines thesource and drain regions, and an additional active-region implant of asecond conductivity type extends over a portion of the gate. Theadditional active-region implant affects the threshold voltage of theaffected portion of the gate; consequently, the gate exhibits twoseparate threshold voltages. The overall threshold voltage of the teststructure is a function of the separate threshold voltages of the gate.

Misalignment of the active region implant in the gate changes therelative areas of the implanted and non-implanted portions of the gate,and consequently changes the overall threshold voltage of the teststructure. The overall threshold voltage increases for misalignment inone direction and decreases with misalignment in the opposite direction.The threshold voltage of the test structure therefore provides a measureof alignment.

Another embodiment of the invention improves measurement accuracy byincluding a second test structure that mirrors the first test structure.Misalignment affects the two test structures in opposite ways, so therespective threshold voltages can be compared to determine the directionof misalignment. Process variations unrelated to alignment generallyaffect both test structures in the same way and therefore tend to cancelout.

Comparing the threshold voltages of two mirrored structures indicateswhether and in what direction the active-region implants are misaligned;however, the difference between the threshold voltages can be difficultto correlate to an extent of misalignment. Another embodiment of theinvention addresses this problem with an array of matchingtest-structure pairs. Each pair differs from the others in the relativeareas of the implanted and non-implanted gate portions. When perfectlyaligned, the threshold voltages of the test structures in each pairmatch, but the threshold voltages differ from one pair to the next.Misalignment causes the threshold voltages within each pair of teststructures to diverge. The collection of diverging threshold-voltagevalues can be used to accurately determine the direction and extent ofmisalignment.

The structures and methods described above are easily adapted for use inmeasuring the alignment of threshold-voltage implants with respect toactive regions. In one such embodiment, special threshold-voltageimplants that extend beneath only a portion of the gate of an MOStransistor cause the transistor to have two channel areas with differentthreshold voltages. The resulting MOS transistor is much the same as theabove-described test structure, and is similarly employed in pairs andarrays to detect and measure misalignment.

This summary does not purport to define the invention. The invention isdefined by the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a plan view of mask regions used to define a conventional MOStransistor.

FIG. 1B depicts an MOS transistor 100 in cross section, taken along lineA-A′ of FIG. 1A.

FIG. 2A is a plan view of an inventive mask-alignment detectionstructure 200 that may be used to measure the extent to whichactive-region implants are aligned with the windows that define theboundaries of the respective active regions.

FIG. 2B schematically depicts structure 200 as a pair ofparallel-coupled MOS transistors 201 and 203.

FIGS. 2C and 2D depict a test structure 250 similar to test structure200 of FIGS. 2A and 2B, like-numbered elements being the same.

FIGS. 3A, 3B, 3C, and 3D depict the same test structures 200 and 250described above in connection with FIGS. 2A-2D.

FIG. 4A depicts a mask-alignment measurement structure 400 that includesan array of structures 405, 410, 415, and 420.

FIG. 4B is a graph depicting the relationship between overlap length OLand threshold voltage V_(T) for the various test structures ofmask-alignment measurement structure 400.

FIG. 5A depicts a mask-alignment measurement structure 500 similar tomask-alignment measurement structure 400 of FIG. 4A.

FIG. 5B is a graph depicting the relationship between intended overlaplength OL_(IN) and the measured threshold voltage V_(T) for the variousright- and left-hand test structures of FIG. 5A.

FIG. 6A depicts a portion of an integrated circuit 600 that includes atest structure 605, in accordance with the invention, along with theconventional transistor 100 depicted in FIGS. 1A and 1B.

FIG. 6B is a cross section of transistors 620 and 625 taken along lineK-K′ of FIG. 6A.

DETAILED DESCRIPTION

FIG. 2A is a plan view of an inventive mask-alignment detectionstructure 200 that may be used to determine whether active-regionimplants are aligned with the windows that define the boundaries of therespective active regions; FIG. 2B schematically depicts structure 200as a pair of parallel-coupled MOS transistors 201 and 203.

Structure 200 includes a window 205 that, like window 105 of FIG. 1A,defines an active region. A gate 207 and an underlying gate insulator(not shown) are conventionally patterned over window 205. A pair ofactive-region implants 210 and 215 defines respective source and drainregions of a first conductivity type (e.g., n-type) on either side of achannel region 212. An additional active-region implant 220 of a secondconductivity type (e.g., p-type) extends over all or a portion of gate207. In the depicted embodiment, active-region implant 220 extends abouthalf way across window 205 over channel region 212, resulting in a firstgate region of a length B and a second gate region of a length C. Thedifferent doping levels of gate 207 divide structure 200 into twoparallel transistors 201 and 203, as illustrated by an imaginary line225.

Active-region implants 210 and 215 are formed during the processsequence used to form other active-region implants on the integratedcircuit. For example, n-type active-region implants 210 and 215 may beformed using the same mask used to form the active regions of NMOStransistors located elsewhere on the integrated circuit. Active-regionimplant 220 is likewise created during the process sequence used to formother active-region implants. For example, a p-type active-regionimplant 220 may be formed using the same mask used to form the activeregions of PMOS transistors located elsewhere on the integrated circuit.Misalignment of active-region implants 210, 215, and 220 indicatessimilar misalignment for structures formed using the same masks.

Structure 200 includes features 235 and 240. As indicated by theirsimilar borders, features 235 and 240 are created using the respectivemasks for active-region implant 220 and window 205. Features 235 and 240are not actual features of the invention, but instead serve toillustrate that in the example of FIG. 2A the masks used to form thevarious layers are perfectly aligned in the X dimension (i.e.,misalignment M=0). Similar features illustrate the extent ofmisalignment for layers depicted in other figures.

The threshold voltage V_(T) of an n-type MOS transistor depends, inpart, on the work function of the material used to form the gate, andthe work function of the gate depends on the dopant type andconcentration in the gate. Increasing the work function of the gate, asby adding p-type dopants, increases the threshold voltage. Conversely,reducing the work function of the gate, as by adding n-type dopants,reduces the threshold voltage. Thus, if we assume that active-regionimplants 210 and 215 are n-type, and that active-region implant 220 isp-type, then the threshold voltage V_(T1) of transistor 201 is lowerthan the threshold voltage V_(T2) of transistor 203 (i.e.,V_(T1)<V_(T2)). The threshold voltage V_(TC1) of structure 200 issomewhere between V_(T1) and V_(T2), affected by the contributions ofboth transistors. (P-type MOS transistors are similarly affected bygate-dopant type and concentration, except that adding n-type dopants tothe gate increases the absolute threshold voltage and adding p-typedopants decreases the absolute threshold voltage.)

FIGS. 2C and 2D depict a test structure 250 similar to test structure200 of FIGS. 2A and 2B, like-numbered elements being the same. Teststructure 250 is formed on the same integrated circuit as test structure200, and includes a second active-region implant 255 that mirrorsactive-region implant 220 of FIG. 2A. That is, active-region implant 255extends across the respective window 205 in a direction opposite toactive-region implant 220, producing an overlap length D betweenactive-region implant 255 and the underlying window 205. The differentdoping levels of gate 207 divide structure 250 into parallel transistors265 and 270, is illustrated by an imaginary line 260.

Overlap length C of FIG. 2A is identical to overlap length D of FIG. 2C(i.e., C=D). Transistors 203 and 265 are therefore identical, and shouldhave the same threshold voltages (i.e., V_(T2)=V_(T3)). Transistors 201and 270 are also identical—assuming the widths of windows 205 areequal—and therefore have the same threshold voltages (i.e.,VT_(T1)=V_(T4)). Further, the threshold voltage V_(TC2) of the combinedtransistors 265 and 270 equals the threshold voltage V_(TC1) of thecombined transistors 201 and 203. As explained below in detail, anymisalignment of the mask used to form active-region implants 220 and 255with respect to the mask used to form windows 205 upsets this balance bychanging the respective current drive of each of transistors 201, 203,265, and 270. The changed threshold voltages V_(TC1) and V_(TC2) canthen be measured and compared to determine whether and in what directionactive-region implants 220 and 255 are misaligned with windows 205.

FIGS. 3A, 3B, 3C, and 3D depict the same test structures 200 and 250described above in connection with FIGS. 2A-2D. FIGS. 3A-3D differ inthat active-region implant 220, active-region implant 255, and feature235 are misaligned with respect to windows 205 and feature 240 by amisalignment Δ. As a consequence of this misalignment, the gate regionsof transistors 201 and 265 are wider and the gate regions of transistors203 and 270 are narrower by misalignment Δ, as compared to theircounterparts in FIGS. 2A-2D.

The change in gate width does not change the threshold voltages of thevarious transistors, but the transistors with wider gates play a largerrole in determining the respective overall threshold voltages V_(TC1)and V_(TC2) of structures 200 and 250. Thus, whereas threshold voltagesV_(TC1) and V_(TC2) were equal in the configuration of FIGS. 2A and 2C,threshold voltages V_(TC1) and V_(TC2) differ in the configuration ofFIGS. 3A and 3C. This difference indicates misalignment.

The wider transistor 201 in structure 200 has a lower threshold voltageV_(T1) than the narrower transistor 203. Thus, the respective increaseand decrease in the channel widths of transistors 201 and 203 reducesthe overall threshold voltage V_(TC1) of structure 200, as compared withthe properly aligned structure of FIG. 2A. In contrast, the widertransistor 265 in structure 250 has a higher threshold voltage V_(T3)than the narrower transistor 270. Thus, the respective increase anddecrease in the channel widths of transistors 265 and 270 change theoverall current through transistor 250 at some gate voltage, andincrease the overall threshold voltage V_(TC2) of structure 250. Thefact that threshold voltage V_(TC1) is less than threshold voltageV_(TC2) indicates that active-region implants 220 and 255 are misalignedwith respect to windows 205 in the positive X direction (the effect isopposite if active-region implants 220 and 255 are n-type).

Comparing the threshold voltages V_(TC1) and V_(TC2) of structures 200and 250 indicates whether and in what direction the active-regionimplants are misaligned with the windows defining the active regions;however, the difference between the threshold voltages V_(TC1) andV_(TC2) can be difficult to correlate to an extent of misalignment. FIG.4A depicts a mask-alignment measurement structure 400 that addressesthis problem. Structure 400 includes an array of test structures 405,410, 415, and 420, each of which includes a pair of test structuressimilar to structures 200 and 250 of FIGS. 2A through 3D, where likeelements are designated using the same reference numbers.

Referring first to the upper left-hand corner of FIG. 4A, a teststructure 405L (the left-hand test structure within structure 405) isdesigned so that active-region implant 220 extends to the left beyondthe boundary of window 205 by an overlap length OL_(L) of 0.3 μm, and tothe right beyond the boundary of window 205 by an alignment tolerance T.Tolerance T is some amount greater than the maximum expectedmisalignment in the X direction. Likewise, referring now to the upperright-hand corner of FIG. 4A, a test structure 405R is designed so thatthe respective active-region implant 255 extends to the right beyond theboundary of window 205 by an overlap length OL_(R) of 0.3 μm, and to theleft beyond the boundary of window 205 by tolerance T.

A second pair of test structures 410L and 410R is identical to the pairconsisting of test structures 405L and 405R, except that the respectiveleft and right overlap lengths OL_(L) and OL_(R) are reduced to 0.1 μm.A third pair of test structure 415L and 415R is identical to the abovetest structures, except that the respective left and right overlaplengths OL_(L) and OL_(R) are reduced to −0.1 μm. Finally, a fourth pairof test structure 410L and 410R is also identical to the above teststructures, except that the respective left and right overlap lengthsOL_(L) and OL_(R) are again reduced, this time to −0.3 μm. The number ofpairs of test structures is reduced for simplicity. In one embodiment,the right and left overlap lengths range from 1 μm to −1 μm inincrements of 0.1 μm.

The left-hand test structures precisely mirror their right-handcounterparts; consequently, the threshold voltages of the left-hand teststructures are identical to the threshold voltages of the respectiveright-hand side test structures (e.g., the threshold voltage ofstructure 405L equals the threshold voltage of structure 405R). Thethreshold voltages of the pairs differ from one another, however, due tothe different overlap lengths between their respective active-regionimplants and windows 205.

FIG. 4B is a graph depicting the relationship between overlap length OLand threshold voltage V_(T) for the various test structures ofmask-alignment measurement structure 400 of FIG. 4A. Structures 405L and405R have the same overlap length (i.e., 0.3 μm) and therefore haveidentical threshold voltages. These identical values are plotted on acurve 425 as the “x” at the far right. Likewise, the relative thresholdvoltages of each successive pair of test structures are plotted on curve425. Curve 425 is illustrative: actual threshold voltages are not givenbecause they vary with process variations.

FIG. 5A depicts a mask-alignment measurement structure 500 similar tomask-alignment measurement structure 400 of FIG. 4A. Structure 500differs from structure 400 in that each active-region implants 220 and255 are misaligned with respect to windows 205 by an alignment error Δof 0.1 μm in the positive X direction. Due to this misalignment, theoverlap length OL_(L) of each left-hand side test structure is reducedby 0.1 μm and the overlap length OL_(R) of each right-hand side teststructure is increased by 0.1 μm. Features 235 and 240 at the bottom ofFIG. 5A indicate the extent of misalignment M.

For illustrative purposes, misalignment M is assumed to be 0.1 μm. Inpractice, the error M is not known, but is to be determined. The“intended” overlap is known, as the intended overlaps are specified inthe layout used to fabricate structure 500 and the rest of theintegrated circuit. The intended overlap lengths OL_(IN) for structures405(L,R), 410(L,R), 415(L,R), and 420(L,R) are 0.3, 0.1, −0.1, and −0.3μm, as depicted in FIG. 4A.

FIG. 5B is a graph depicting the relationship between intended overlaplength OL_(IN) and the measured threshold voltage V_(T) for the variousright- and left-hand test structures of FIG. 5A. The graph includesthree curves 425, 510, and 515. Curve 425 is the ideal curve taken fromFIG. 4B, in which active-region implants 220 and 225 are preciselyaligned with windows 205. Square data points represent threshold-voltagedata taken from test structures selected from the left-hand side ofstructure 500 (identified with the suffix “L”); circular data pointsrepresent resistance data taken from test structures selected from theright-hand side of structure 500 (identified with the suffix

The threshold voltages V_(T) of the right-hand test structures aregenerally increased and the threshold voltages V_(T) of left-hand teststructures are generally decreased due to the misalignment.Consequently, curve 510, drawn through the circular data points, issimilar to the ideal curve but shifted to the left by 0.1 μm, themisalignment M. Likewise, curve 515, drawn through the square datapoints, is similar to ideal curve 425 but shifted to the right 0.1 μm.Misalignment M is calculated by measuring the offset of curves 510 and515 in the X dimension and dividing the result by two. These resultsassume that active-region implants 220 and 255 increase the workfunction of the affected regions of respective gates 207.

The shapes of curves 510, 425, and 515 can change due to misalignment inthe Y dimension and other process variations. However, the spacingbetween curves 510 and 515 in the x dimension (twice the misalignment M)is relatively independent of these factors, provided the processvariations are not extreme. Thus, structure 500 produces an accuratemeasure of both the extent and direction of misalignment.

The following Table 1 illustrates how hypothetical data—obtained usingan exemplary misaligned structure similar to structure 500 of FIG. 5A—isused to measure misalignment. Threshold voltages V_(TO) through V_(T3)are hypothetical. The first column, labeled “Error,” represents anamount of misalignment M between active-region implants 220 255 andwindows 205; in other words, between the active-region implants and thecorresponding active regions. In this example, the error M is positivewhen implants 220 and 255 shifted to the right (i.e., the positive Xdirection) respect to windows 205.

TABLE 1 ERROR 405 410 415 420 (μm) SIDE (0.3) (0.1) (−0.1) (−0.3) M = 0L V_(T0) V_(T1) V_(T2) V_(T3) R V_(T0) V_(T1) V_(T2) V_(T3) M = 0.2 LV_(T1) V_(T2) V_(T3) R V_(T0) V_(T1) V_(T2) M = −0.2 L V_(T0) V_(T1)V_(T2) R V_(T1) V_(T2) V_(T3)

The two rows labeled M=0 show that the threshold voltages correspondingto L and R (the respective left- and right-side test structures) areequal for each of structures 405, 410, 415, and 420. The rows labeledM=0.2 show that for a misalignment of 0.2 μm the threshold voltagescorresponding the left-side structures decrease and the thresholdvoltages of the right side increase so that equivalent thresholdvoltages are offset by 0.4 μm, or 2 M. For example, threshold voltageV_(T1) is associated with the left side of structure 405 and the rightside of structure 415. These structures were designed to have overlapsthat differ by 0.4 μm; the fact that they exhibit the same thresholdvoltage indicates that their respective active-region implants areshifted 0.2 μm with respect to conductive windows 205. The equivalentthreshold voltages V_(T2) associated with the left-hand test structure410L and the right-hand test structure 420R indicate the same degree ofmisalignment. The fact that the threshold voltages for the left-handtest structures are lower than those of the right-hand test structuresindicates that active-region implants 220 and 255 are misaligned to theright. Finally, the rows labeled M=−0.2 show that for a misalignment of−0.2 μm the threshold voltages corresponding the left-side structuresincrease and the threshold voltages of the right side decrease so thatequivalent threshold voltages are offset by −0.4 μm, or 2 M. The factthat the left-hand test structures exhibit higher threshold voltagesthan do the right-hand test structures indicates that active-regionimplants 220 and 255 are misaligned to the left with respect to windows205.

The structures and methods described above are easily adapted for use inmeasuring the alignment of threshold-voltage implants with respect toactive regions. Referring to FIGS. 1A and 1B, for example, someembodiments of the invention can be used to ensure thatthreshold-voltage implant 112 is aligned with window 105, the boundaryof an active region.

The threshold voltage V_(T) of an MOS transistor depends, in part, onthe dopant type and concentration in the channel region. For example,adding p-type dopants to the channel region of an n-type transistorincreases the threshold voltage, and adding n-type dopants to thechannel region of an n-type transistor decreases the threshold voltage.The dependence of threshold voltage upon channel doping is well known tothose skilled in the art.

FIG. 6A depicts a portion of an integrated circuit 600 that includes atest structure 605 in accordance with the invention along with theconventional transistor 100 depicted in FIGS. 1A and 1B. FIG. 6Aincludes a plan view of the feature boundaries of transistor 100 to showthe relative alignment of window 105, threshold-voltage implant boundary111, active-region implant boundary 121, and gate 115. IC 600 alsoincludes imaginary features 610 and 615 to illustrate thatthreshold-voltage implant boundary 111 is aligned in a Y dimension withwindow 105. This alignment is desirable, as the threshold-voltageimplant (defined within boundary 111) should alter the doping level inthe channel region, the area of window 105 directly below thesubsequently formed gate 115.

Test structure 605 includes a pair of transistors 620 and 625.Transistors 620 and 625 are substantially identical, but intentionallydiffer in the alignment their respective threshold-voltage implantregions 630A and 630B. Threshold-voltage implant region 630A extendsinto the respective window 105 of transistor 620 in the positive Ydirection, whereas threshold-voltage implant region 630B extends intothe respective window 105 of in the negative Y direction. Thusconfigured, misalignment of the mask used to form threshold-voltageimplant regions 111, 630A, and 630B with respect to the mask used toform windows 105 will have substantially the same effect on thethreshold voltages of both transistors 620 and 625 if the misalignmentis in the x dimension, but will have opposite effects on transistors 620and 625 if the misalignment is in the Y dimension. The presence anddirection of misalignment in the Y dimension can therefore be determinedby comparing the threshold voltages of transistors 620 and 625.

FIG. 6B is a cross section of transistors 620 and 625 taken along lineK-K′ of FIG. 6A, and illustrates that threshold-voltage adjustmentimplant 630B extends across line K-K′ and threshold-voltage adjustmentimplant 630A does not. Channel region 640A beneath gate 115 oftransistor 620 includes a first active region 645 (FIG. 6A) having firstdopant concentration and a second active region 650 having a seconddopant concentration determined, in part, by threshold-voltage implantregion 630A. Channel region 640B of transistor 625 similarly includestwo active regions, a third active region 655 having the second dopantconcentration and a fourth active region 660 having the first dopantconcentration.

Transistors 620 and 625 detect misalignment in much the same way asstructures 200 and 250 of FIGS. 2A through 5B. The threshold voltages oftransistors 620 and 625 depend upon the channel doping of respectivechannel regions 640A and 640B, and therefore upon the extent to whichthreshold-voltage implants 630A and 630B overlap windows 105. In thecase of perfect alignment in the Y dimension, the threshold voltages oftransistors 620 and 625 are identical; the respective threshold voltagesdiffer in proportion to the extent of misalignment in the Y dimension.Arrays of transistor pairs having incremental changes in the extent ofoverlap can be used as described above in connection with FIGS. 4A-5B toaccurately measure the extent and direction of misalignment between thethreshold-voltage implants and the active regions defined within windows105.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, the embodiments ofFIGS. 4A and 4B are illustrated as having four pairs of test structures.Actual circuit implementations can include many more, and semiconductorwafers might include many arrays of such test structures. Further, eachof the above-described structures measures misalignment in onedimension. Similar structures oriented in other dimensions detectmisalignment in other directions. Therefore, the spirit and scope of theappended claims should not be limited to the foregoing description.

What is claimed is:
 1. A method of determining an extent to which anactive diffusion pattern of an integrated circuit is aligned in a planewith active regions of the integrated circuit, the method comprisingforming a plurality of transistors, each having a gate partiallyimplanted using the active diffusion pattern to form a first gate regionhaving a first work function and a second gate region having a secondwork function, wherein the threshold voltage of each transistor variesin proportion to an extent of misalignment, in one direction, betweenthe active diffusion pattern and the active regions.
 2. The method ofclaim 1, further comprising measuring the respective threshold voltagesof the transistors.
 3. The method of claim 1, further comprising forminga second plurality of transistors, each having a gate partiallyimplanted using the active diffusion pattern to form a third gate regionhaving the second work function and a fourth gate region having thefirst work function, wherein the threshold voltage of each of the secondplurality of transistors varies in inverse proportion of the extent ofmisalignment in the one direction.
 4. A method of determining an extentto which an implant region of an integrated circuit is aligned in aplane with an active region of the integrated circuit, the methodcomprising: a. forming a first test structure including: i. a firstsource region; ii. a first drain region; iii. a first channel region ofa first area between the first source and drain regions and having afirst threshold voltage; and iv. a second channel region of a secondarea between the first source and drain regions and having a secondthreshold voltage different from the first threshold voltage; v. whereinthe first and second areas depend upon an extent of misalignment betweenthe implant region and the active region in a first dimension parallelto the plane, the first area increasing and the second area decreasingwith misalignment in one direction in the first dimension; and b.forming a second test structure including: i. a second source region;ii. a second drain region; iii. a third channel region of the first areabetween the second source and drain regions and having the secondthreshold voltage; and iv. a fourth channel region of the second areabetween the second source and drain regions and having the firstthreshold voltage.
 5. The method of claim 4, further comprising forminga first gate over the first and second channel regions and forming asecond gate over the third and fourth channel regions.
 6. The method ofclaim 5, further comprising: a. measuring a fifth threshold voltage ofthe first structure by applying a first voltage signal to the firstgate, the fifth threshold voltage being a function of the first andsecond threshold voltages; and b. measuring a sixth threshold voltage ofthe second structure by applying a second voltage signal to the secondgate, the sixth threshold voltage being a function of the third andfourth threshold voltages.
 7. The method of claim 6, further comprisingcomparing the fifth and sixth threshold voltages.
 8. A method ofmeasuring an extent of misalignment between a first plurality of implantand a respective plurality of implant regions, the method comprising: a.forming a first plurality of transistors, each transistor having athreshold voltage that varies in proportion to the extent ofmisalignment; b. forming a second plurality of transistors, eachtransistor having a threshold voltage that varies in inverse proportionto the extent of misalignment; and c. measuring the threshold voltagesof each of the first and second pluralities of transistors.
 9. Themethod of claim 8, wherein forming the first plurality of transistorscomprises forming a gate structure for each of the first plurality oftransistors, each gate structure having a first gate region of a firstwork function and a second gate region of a second work functiondifferent from the first work function.
 10. The method of claim 9,wherein forming the second plurality of transistors comprises forming agate structure for each of the second plurality of transistors, eachgate structure having a third gate region of the first work function anda fourth gate region of the second work function different from thefirst work function.
 11. The method of claim 8, wherein forming thefirst plurality of transistors comprises forming a channel region foreach of the first plurality of transistors, each channel region having afirst active region of a first channel-dopant concentration and a secondactive region of a second channel-dopant concentration different fromthe first channel-dopant concentration.
 12. The method of claim 11,wherein forming the second plurality of transistors comprises forming achannel region for each of the second plurality of transistors, eachchannel region having a third active region of the first channel-dopantconcentration and a fourth active region of the second channel-dopantconcentration.